Nuclear magnetic resonance (NMR) measurements typically are performed to investigate properties of a sample. For example, an NMR wireline or logging while drilling (LWD) downhole tool may be used to measure properties of subterranean formations. In this manner, the typical downhole NMR tool may, for example, provide a lithology-independent measurement of the porosity of a particular formation by determining the total amount of hydrogen present in fluids of the formation. Equally important, the NMR tool may also provide measurements that indicate the dynamic properties and environment of the fluids, as these factors may be related to petrophysically important parameters. For example, the NMR measurements may provide information that may be used to derive the permeability of the formation and viscosity of fluids contained within the pore space of the formation. It may be difficult or impossible to derive this information from other conventional logging arrangements. Thus, it is the capacity of the NMR tool to perform these measurements that makes it particularly attractive versus other types of downhole tools.
Typical NMR logging tools include a magnet that is used to polarize hydrogen nuclei (protons) in the formation and a transmitter coil, or antenna, that receives radio frequency (RF) pulses from a pulse generator of the tool and in response, radiates RF pulses into the formation. A receiver antenna may measure the response (indicated by a received RF signal called a spin echo signal) of the polarized hydrogen to the transmitted pulses. Quite often, the transmitter and receiver antennae are combined into a single transmitter/receiver antenna. echo signal) of the polarized hydrogen to the transmitted pulses. Quite often, the transmitter and receiver antennae are combined into a single transmitter/receiver antenna.
The NMR techniques employed in current NMR tools typically involve some variant of a basic two step technique that includes delaying for a polarization time and thereafter using an acquisition sequence. During the polarization time (often referred to as a xe2x80x9cwait timexe2x80x9d), the protons in the formation polarize in the direction of a static magnetic field (called B0) that is established by a permanent magnet (of the NMR tool).
An example of an NMR sequence is a Carr-Purcell-Meiboom-Gill (CPMG) sequence 15 that is depicted in FIG. 1. By applying the sequence 15, a distribution of spin relaxation times (T2 times, for example) may be obtained, and this distribution may be used to determine and map the properties of a formation. A technique that uses CPMG sequences 15 to measure the T2 times may include the following steps. In the first step, the NMR tool pulses an RF field (called the B1 field) for an appropriate time interval to apply a 90xc2x0 excitation pulse 14a to rotate the spins of hydrogen nuclei that are initially aligned along the direction of the B0 field. Although not shown in detail, each pulse is effectively an envelope, or burst, of an RF carrier signal. When the spins are rotated around B1 away from the direction of the B0 field, the spins immediately begin to precess around B0. At the end of the pulse 14a, the spins are rotated by 90xc2x0 into the plane perpendicular to the B0 field. The spins continue to precess in this plane first in unison, then gradually losing synchronization.
For step two, at a fixed time TCP following the excitation pulse 14a, the NMR tool pulses the B1 field for a longer period of time (than the excitation pulse 14a) to apply an NMR refocusing pulse 14b to rotate the precessing spins through an angle of 180xc2x0 with the carrier phase shifted by xc2x190xc2x0. The NMR pulse 14b causes the spins to resynchronize and radiate an associated spin echo signal 16 (see FIG. 2) that peaks at 2xc2x7TCP after the 90xc2x0 tipping pulse 14a. Step two may be repeated xe2x80x9ckxe2x80x9d times (where xe2x80x9ckxe2x80x9d is called the number of echoes and may assume a value anywhere from several to as many as several thousand, as an example) at the interval of 2xc2x7TCP. For step three, after completing the spin-echo sequence, a waiting period (usually called a wait time) is required to allow the spins to return to equilibrium along the B0 field before starting the next CPMG sequence 15 to collect another set of spin echo signals. The decay of the amplitudes of each set of spin echo signals 16 may be used to derive a distribution of T2 times.
Although it may be desirable to vary the characteristics of the measurement sequence to optimize performance to a particular formation, unfortunately, a conventional NMR tool may be specifically designed to perform a predefined NMR measurement sequence. Thus, the conventional tool may provide limited flexibility for changing the sequence, as the parameters that may be programmed into the tool may affect the global timing of the sequence without allowing the flexibility to change a particular portion of the sequence. For example, a conventional NMR tool may be programmed with the above-described TCP time, the time between the tipping pulse 14a and the first refocusing pulse 14b. However, this value also sets the time (2xc2x7TCP) between successive refocusing pulses 14b. Thus, although a time between refocusing pulses 14b other than 2xc2x7TCP may be desired to optimize performance of the tool, the tool may not provide the flexibility to change this time.
The subject invention is an apparatus and method for tuning an NMR coil. The method comprises the steps of using a coil to perform NMR measurements. Between two of the measurements, pulsing the coil with an RF burst and monitoring a voltage of the coil after the RF burst. Based on the monitored voltage, tuning the coil before performing the next NMR measurement.
The apparatus comprises a coil and a circuit coupled to the coil. The circuit is adapted to use the coil to perform NMR measurements. Between two of the measurements, the coil is pulsed with an RF burst. The voltage of the coil is monitored after the RF burst. Based on the monitored voltage, the coil is tuned before performing the next NMR measurement.